As described in provisional patent application 61/486,956 filed May 17, 2011, it is desirable to provide a thin structure for an antenna embedded in an armor panel and more particularly to provide a parasitic element on top of the armor layer so that when driving the antenna there are no apertures in the armor which would degrade performance. In one embodiment the aperture-less embedded antenna system includes a direct fed dipole on the underneath side of the armor layer such that the armor layer is not pierced. There is an identical dipole on the top of the armor layer that is parasitically fed by the driven dipole. In one embodiment the dipoles are in the form of bowties.
As described in the above-identified provisional patent application, it is desirable to replace antennas such as whip antennas on tanks, armored vehicles and the like with broadband antennas that are conformal to the vehicle itself.
Having a forest of antennas that extend from the armored vehicle is undesirable because they are susceptible to damage and attack. It is therefore desirable to be able to provide an antenna system which is embedded in the armor such that the armor protects the embedded antenna both against explosive attacks and ballistic penetration while at the same time eliminating the need for antenna whips, dashes and the like which are easily blown off with explosive charges, thereby precluding communication with the vehicle.
It is noted that the thin structure of present armor panels presents the greatest challenge to antenna design. Whether the panel is metal backed itself or is mounted on a metal vehicle, the close proximity of a conductive surface to a radiating element creates a ground plane that is too close to the element. As will be appreciated in traditional antenna design, the ground plane is spaced at least a quarter wavelength away from any driven element. However, when dealing with armor for vehicles such as tanks and the like, the spacing between the ground plane and the driven element of the antenna is on the order of hundredths of a wavelength.
While initially thought that this limitation would be a disqualifying factor in the antenna design, it has been shown that a thin antenna structure can be created which does not rely on deep cavities behind the elements. Such structures have been described in U.S. Pat. No. 6,833,815 which relates to Cavity Embedded Meanderline Loaded Antennas. In this patent the antenna described is a conformal antenna which is cavity-backed.
In one embodiment of this Cavity Embedded Meanderline Antenna a bowtie dipole is utilized, with the distal ends of the dipole being coupled to surrounding metal utilizing a meanderline structure.
Since it is possible to completely quantify the electromagnetic characteristics of the armor materials one can establish the permittivity and loss of each piece of the armor recipe that affects the effective electrical length and efficiency of the radiating structure. This being said, it was thought that the dielectric constants of overlying or intermediate materials could be tailored to reduce VSWR and maximize gain. It was thought that this could be accomplished by completely characterizing the boundaries between the layers within the armor as well as the boundary to the outside or free space.
While the presence of a dielectric allows one to accommodate the thin armor structure, it has been found that regardless of the dielectric matching a thin stacked element array is achievable using a driven bowtie dipole to the inside of an alumina tile armor plate and a parasitic element in the form of an identical parasitically driven bowtie is on the outside of the armor plate. As discussed in this provisional patent application, it is possible to use an embedded driven element and an outer parasitic element approach that does not depend heavily on impedance matching layers.
More specifically it was found that by utilizing the parasitic element on top of the armor plate and by driving it with a driven element beneath the armor plate, satisfactory antenna performance can be obtained in the 225-450 MHz range.
More particularly, when utilizing a parasitically-driven array in which the driven element is beneath the armor layer and the parasitically-driven element is above or to the outside of the armor layer, it was found that one can have unity gain across the 225-450 MHz range with a VSWR of 3:1 or less across the range.
There is however a problem in extending the range of such an armor-embedded antenna for wideband to cover for instance 30 MHz to 455 MHz. It will be appreciated that if a single wideband antenna could be embedded in the armor, then one can have a wide range of communications options without having a forest of antennas each tuned to a separate frequency band and each vulnerable to attack.